The present invention relates to shell and tube heat exchangers and, in particular, to an arrangement for marking specific tubes, for example tubes that need to be repaired or otherwise corrected. Shell and tube heat exchangers that are used as chemical reactors may have many thousands of open-ended tubes. Various methods have been used in the past to mark the tubes that need to be corrected. Usually, a marker has been inserted into the open end of the tube that needs to be corrected, but those markers can come loose when people walk on top of the tube sheet, and they require the marker to be removed in order to treat the tube. Also, when the marker is removed in order to correct the tube, the location of the tube may be lost.